1. Technical Field
This invention relates to metal gaskets and to methods of their manufacture.
2. Related Prior Art
Metallic gaskets are widely used in internal combustion engine applications to seal the gap between the joining deck surfaces of the cylinder heads and block to prevent leakage of fluids such as combustion gases, oil, coolant, and the like. The gaskets typically are constructed from several individual metal layers formed with the appropriate openings and sealing features. The layers are laminated to yield a resiliently compressible, multi-layer structure. The outer active layers are typically formed with annular embossed beads that are compressed between the head and block and undergo elastic deformation to assure that a seal is maintained.
The material for the active layers must be ductile enough to allow for initial formation of the beads, yet sufficiently hard and strong in use to withstand considerable loading and deformation without cracking or yielding plastically. Operating conditions of an engine can also produce large planar (lateral) motions and major vertical deformations of the gasket due to thermal changes, particularly during engine start-up. Ongoing combustion can produce both rapid cyclic lateral and vertical movements of the gasket.
Roll hardened stainless steels of the 300 series, and principally 301 full hard stainless, is often used as the material for the active layers. The full hard stainless materials possess the hardness and strength required for the active gasket layers. However, beginning with a full hard material presents difficulties in forming the sealing beads. The thickness of the material and the height of the bead embossments is limited because of the larger press tonnages required to form the embossments. Consequently, full hard active layers are typically made very thin (i.e., on the order of about 0.010xe2x80x3) and the embossments modest, requiring multiple layers to make up the needed thickness and compressibility of the gasket. Furthermore, deforming such full hard material introduces localized residual forming stresses and increases the hardness in the vicinity of the embossments, which can contribute to fatigue failure of the gasket if not controlled.
Some gaskets incorporate a stopper to limit the compression of the beads. The full hard materials used for the active layers are generally considered too hard to accommodate the formation of deformed stopper features in the full hard active layers. The stopper features are often formed in a separate stopper layer fabricated typically of low carbon steel, which is readily formable to accommodate the stopper, but has insufficient hardness and strength needed for an active layer.
U.S. Pat. No. 4,721,315 discloses a multi-layer gasket that is silent as to the layer materials. The construction employs a base layer formed with embossed bead and includes another layer attached to the base layer that serves as a spacer (stopper). U.S. Pat. No. 4,196,913 discloses an early multi-layer gasket having layers formed by punching a hard, thin metal plate such as a stainless, carbon or spring steel. There is no disclosure as to how the materials are hardened, other than the disclosure that the materials are hard at the time of punching the embossments or undulations in the gasket layers. Presumably, the materials are cold rolled hardened sheet stock. Such pre-hardened materials would share the same difficulties and limitations discussed above with respect to the current full hard stainless materials.
U.S. Pat. No. 5,310,196 discloses a single layer metal gasket fabricated of specified materials that can be formed in their soft state and then heat treated to achieve a desired hardness. The disclosure is silent as to how the heat treatment processes could be carried out in a way that would preserve the dimensional stability of the gasket layers, which by their nature are extremely thin and have considerable surface area that would make them subject to warpage if heat treated improperly. Specifically, the disclosure does not teach or suggest austempering steels as a viable, dimensionally stable alternative to roll hardened stainless steel gasket layers.
According to the invention, a metal gasket is provided having at least one layer formed with a plurality of openings and sealing bead deformations surrounding at least one of the openings. The gasket is characterized by the layer being fabricated from austemper-hardenable steel having a generally bainitic microstructure.
According to a method of the invention for forming metal gaskets having at least one layer fabricated from austemper-hardenable steel, the layer is first formed with a plurality of openings and deformed to include sealing beads when the material is in a relatively soft, pre-heat treat hardened condition, and thereafter the gasket layer is austempered to achieve a generally bainitic microstructure harden and strengthen the material while maintaining dimensional stability of the layer.
The invention has the advantage of providing a practical, viable alternative to the use of conventional roll hardened stainless steel gasket layers. Austempering makes possible and practical the use of an inexpensive family of steels that can be formed when in a soft condition to decrease the tonnages and cost of forming beads. Following deformation, the material can be austempered to achieve a high hardness and strength comparable or exceeding that of conventional roll hardened materials and a fatigue strength that is considerably greater (fatigue strength equal to about xc2xd of tensile strength for austempered layer as compared to ⅓ the tensile strength for roll hardened stainless steel materials.
The austempered gasket layer may be used alone in a single layer metal gasket application or as one or more layers in a multi-layer gasket application.